The disclosures herein relate generally to multiple fan systems. More particularly, the disclosures herein relate to multiple fan ventilating systems for computers.
In a multiple fan ventilating system, two or more fans running at the same voltage will operate at slightly different revolutions per minute (RPM). The difference in the RPM may be due to conditions such as manufacturing tolerances or the use of two different brands of fans. Regardless of the reason for the difference in the RPM, it will result in an oscillating noise known as a `beat`. The beat occurs when the waveforms associated with the different fan frequencies interact and the waveforms add their amplitudes. An oscillation occurs between the various waveforms where they reinforce each other and then cancel each other. With high frequency devices such as ventilation fans used in computers, this interaction often appears as a background noise in the audio spectrum.
Some countries have instituted acoustical standards that place limits on the magnitude of acoustic noises such as beat frequency oscillations. Two examples of such standards are ISO 7779 and ISO 3745. Both specifications state that `impulsive noises` must be minimized. Beats are a particularly noticeable part of this impulsive noise. The objective of such standardization is to ensure that the acoustical noise does not adversely affect workplace productivity and comfort. Additionally, in instances where a computer is used to capture and process audio information with a microphone, noise associated with a beat frequency may adversely affect performance.
U.S. Pat. No. 5,136,465 to Benck et al discloses a personal computer in which provisions are made for effectively cooling components capable of generating heat during operation. The personal computer has an enclosure for enclosing operating components and a printed circuit board mounted within the enclosure for supporting and interconnecting the operating components. A first fan for inducing air flow into the enclosure and a second fan for expelling air from the enclosure are located within the enclosure. An air flow baffle directs the air flow through the enclosure to pass over and cool the heat generating components. No technique or components for reducing induced noise in a multiple fan system is disclosed.
U.S. Pat. No. 5,791,869 to Lee discloses a noise killing system in which a micro-electro mechanical system is mounted on the body of a fan blade. The micro-electro mechanical system includes at least one thin silicon film forming an integrated circuit and an actuator connected to the circuit for generating vibrations. To kill noise, the activator generates vibrations which offset (reduce) unstable air along the fan blade. A system or method for oscillating the speed of a fan is not disclosed. Furthermore, this reference does not disclose a multiple fan system.
U.S. Pat. No. 5,546,272 to Moss et al discloses a cooling subsystem and method for a chassis of a computer. The cooling system comprises: (1) first and second cooling fans having first and second motors associated therewith for driving the respective fans and (2) a common plenum substantially shrouded and providing a pathway for air communication between the first and second fans. The first and second fans cooperate to provide an optimum rate of air flow within the chassis. The reference does not disclose a system for oscillating the speed of a fan.
U.S. Pat. No. 5,778,081 to Patrick discloses an active noise control system including sensors which detect noise and provide electronic signals to an active noise control controller. The controller provides electronic anti-noise signals to a speaker which is connected to, and provides acoustic anti-noise into, a plurality of active resonators. The resonators are disposed successively along the propagation direction of the noise and provide time-delayed anti-noise acoustic output signals, each of which attenuates a portion of the noise. This noise control system does not disclose a multiple fan system or means for oscillating the speed of fans. Furthermore, the complexity of this system indicates that it is not a cost-effective solution for cost sensitive applications.
Accordingly, a need has arisen for a multiple fan system in which the shortcomings of the previous techniques are overcome. More particularly, a need has arisen for a multiple fan ventilating system for a computer in which a simple, cost-effective means is provided for minimizing or eliminating noise associated with the beat frequency oscillations of two or more fans operating at different fixed speeds.